Small animal magnetic resonance imaging: an efficient tool to assess liver volume and intrahepatic vascular anatomy

Post mortem analysis of hepatic volume and lipid content by magnetic resonance imaging and spectroscopy in fixed murine neonates

Maternal obesity and hyperglycemia are linked to an elevated risk for obesity, diabetes, and steatotic liver disease in the adult offspring. To establish and validate a noninvasive workflow for perinatal metabolic phenotyping, fixed neonates of common mouse strains were analyzed postmortem via magnetic resonance imaging (MRI)/magnetic resonance spectroscopy (MRS) to assess liver volume and hepatic lipid (HL) content. The key advantage of nondestructive MRI/MRS analysis is the possibility of further tissue analyses, such as immunohistochemistry, RNA extraction, and even proteomics, maximizing the data that can be gained per individual and therefore facilitating comprehensive correlation analyses. This study employed an MRI and 1H-MRS workflow to measure liver volume and HL content in 65 paraformaldehyde-fixed murine neonates at 11.7 T. Liver volume was obtained using semiautomatic segmentation of MRI acquired by a RARE sequence with 0.5-mm slice thickness. HL content was measured by a STEAM sequence, applied with and without water suppression. T1 and T2 relaxation times of lipids and water were measured for respective correction of signal intensity. The HL content, given as CH2/(CH2 + H2O), was calculated, and the intrasession repeatability of the method was tested. The established workflow yielded robust results with a variation of ~3% in repeated measurements for HL content determination. HL content measurements were further validated by correlation analysis with biochemically assessed triglyceride contents (R2 = 0.795) that were measured in littermates. In addition, image quality also allowed quantification of subcutaneous adipose tissue and stomach diameter. The highest HL content was measured in C57Bl/6N (4.2%) and the largest liver volume and stomach diameter in CBA (53.1 mm3 and 6.73 mm) and NMRI (51.4 mm3 and 5.96 mm) neonates, which also had the most subcutaneous adipose tissue. The observed effects were independent of sex and litter size. In conclusion, we have successfully tested and validated a robust MRI/MRS workflow that allows assessment of morphology and HL content and further enables paraformaldehyde-fixed tissue-compatible subsequent analyses in murine neonates.

Gender Differences in a Mouse Model of Hepatocellular Carcinoma Revealed Using Multi-Modal Imaging

The worldwide incidence of hepatocellular carcinoma (HCC) continues to rise, in part due to poor diet, limited exercise, and alcohol abuse. Numerous studies have suggested that the loss or mutation of PTEN plays a critical role in HCC tumorigenesis through the activation of the PI3K/Akt signaling axis. The homozygous knockout of PTEN in the livers of mice results in the accumulation of fat (steatosis), inflammation, fibrosis, and eventually progression to HCC. This phenotype bears a striking similarity to non-alcoholic steatohepatitis (NASH) which is thought to occupy an intermediate stage between non-alcoholic fatty liver disease (NAFLD), fibrosis, and HCC. The molecular and physiological phenotypes that manifest during the transition to HCC suggest that molecular imaging could provide a non-invasive screening platform to identify the hallmarks of HCC initiation prior to the presentation of clinical disease. We have carried out longitudinal imaging studies on the liver-specific PTEN knockout mouse model using CT, MRI, and multi-tracer PET to interrogate liver size, steatosis, inflammation, and apoptosis. In male PTEN knockout mice, significant steatosis was observed as early as 3 months using both magnetic resonance spectroscopy (MRS) and computed tomography (CT). Enhanced uptake of the apoptosis tracer 18F-TBD was also observed in the livers of male PTEN homozygous knockout mice between 3 and 4 months of age relative to heterozygous knockout controls. Liver uptake of the inflammation tracer [18F]4FN remained relatively low and constant over 7 months in male PTEN homozygous knockout mice, suggesting the suppression of high-energy ROS/RNS with PTEN deletion relative to heterozygous males where the [18F]4FN liver uptake was elevated at early and late time points. All male PTEN homozygous mice developed HCC lesions by month 10. In contrast to the male cohort, only 20% (2 out of 10) of female PTEN homozygous knockout mice developed HCC lesions by month 10. Steatosis was significantly less pronounced in the female PTEN homozygous knockout mice relative to males and could not accurately predict the eventual occurrence of HCC. As with the males, the [18F]4FN uptake in female PTEN homozygous knockout mice was low and constant throughout the time course. The liver uptake of 18F-TBD at 3 and 4.5 months was higher in the two female PTEN knockout mice that would eventually develop HCC and was the most predictive imaging biomarker for HCC in the female cohort. These studies demonstrate the diagnostic and prognostic role of multi-modal imaging in HCC mouse models and provide compelling evidence that disease progression in the PTEN knockout model is highly dependent on gender.

A dopamine-precursor-based nanoprodrug for in-situ drug release and treatment of acute liver failure by inhibiting NLRP3 inflammasome and facilitating liver regeneration

Acute liver failure (ALF) is a severe liver disease with high mortality rate. Inflammasome is a newly-found and promising target for effective treatment of immunity-associated diseases including liver disease, and dopamine has recently been proved as an inhibitor for NLRP3 inflammasome. This work demonstrates a diselenide-based nanodrug for ALF treatment through inhibiting NLRP3 inflammasome activation and enhancing liver regeneration. A diselenide-containing molecule (DSeSeD) has been synthesized via covalently linking two l-Dopa molecules to a diselenide linker, and the resultant molecules form stable nanoparticles in aqueous media and encapsulate SW033291 (an inhibitor of prostaglandin-degrading enzyme that hampers liver regeneration) to produce the nanodrug (SW@DSeSeD). As a nanoscale prodrug, SW@DSeSeD protects its payloads from decomposition in bloodstream upon administration, accumulates in liver of ALF mice, then responds to the overexpressed ROS and thereby releases SW033291 as well as a stable dopamine precursor that can transform into dopamine in hepatic cells, thus achieving significant therapeutic efficacy against ALF through inhibiting NLRP3 inflammasome activation and enhancing hepatic regeneration. Moreover, multiple contrast agents have been loaded onto the nanodrug to achieve fluorescence, optoacoustic and magnetic resonance imaging for nanodrug location and disease evaluation.

A characterization of ABL-101 as a potential tracer for clinical fluorine-19 MRI

The two main challenges that prevent the translation of fluorine-19 (¹⁹ F) MRI for inflammation monitoring or cell tracking into clinical practice are (i) the relatively low signal-to-noise ratio generated by the injected perfluorocarbon (PFC), which necessitates long scan times, and (ii) the need for regulatory approval and a high biocompatibility of PFCs that are also suitable for MRI. ABL-101, an emulsion of perfluoro(t-butylcyclohexane), is a third-generation PFC that is already used in clinical trials, but has not yet been used for ¹⁹ F MRI. The objective of this study was therefore to assess the performance of ABL-101 as a ¹⁹ F MRI tracer. At magnetic field strengths of 3, 9.4 and 14.1 T, the CF₃ groups of ABL-101 generated a large well-separated singlet with T₂ /T₁ ratios of >0.27, >0.14 and > 0.05, respectively. All relaxation times decreased with the increase in magnetic field strength. The detection limit of ABL-101 in a 0.25 mm³ voxel at 3 T, 37°C and with a 3-minute acquisition time was 7.21mM. After intravenous injection, the clearance half-lives of the ABL-101 ¹⁹ F MR signal in mouse (n = 3) spleen and liver were 6.85 ± 0.45 and 3.20 ± 0.35 days, respectively. These results demonstrate that ABL-101 has ¹⁹ F MR characteristics that are similar to those of PFCs developed specifically for MRI, while it has clearance half-lives similar to PFCs that have previously been used in large doses in non-MRI clinical trials. Overall, ABL-101 is thus a very promising candidate tracer for future clinical trials that use ¹⁹ F MRI for cell tracking or the monitoring of inflammation.

In vivo Biodistribution of Radiolabeled Acoustic Protein Nanostructures

PURPOSE

Contrast-enhanced ultrasound plays an expanding role in oncology, but its applicability to molecular imaging is hindered by a lack of nanoscale contrast agents that can reach targets outside the vasculature. Gas vesicles (GVs)-a unique class of gas-filled protein nanostructures-have recently been introduced as a promising new class of ultrasound contrast agents that can potentially access the extravascular space and be modified for molecular targeting. The purpose of the present study is to determine the quantitative biodistribution of GVs, which is critical for their development as imaging agents.

PROCEDURES

We use a novel bioorthogonal radiolabeling strategy to prepare technetium-99m-radiolabeled ([^99mTc])GVs in high radiochemical purity. We use single photon emission computed tomography (SPECT) and tissue counting to quantitatively assess GV biodistribution in mice.

RESULTS

Twenty minutes following administration to mice, the SPECT biodistribution shows that 84 % of [^99mTc]GVs are taken up by the reticuloendothelial system (RES) and 13 % are found in the gall bladder and duodenum. Quantitative tissue counting shows that the uptake (mean ± SEM % of injected dose/organ) is 0.6 ± 0.2 for the gall bladder, 46.2 ± 3.1 for the liver, 1.91 ± 0.16 for the lungs, and 1.3 ± 0.3 for the spleen. Fluorescence imaging confirmed the presence of GVs in RES.

CONCLUSIONS

These results provide essential information for the development of GVs as targeted nanoscale imaging agents for ultrasound.

Prediction of small for size syndrome after extended hepatectomy: Tissue characterization by relaxometry, diffusion weighted magnetic resonance imaging and magnetization transfer

This study aimed to monitor the course of liver regeneration by multiparametric magnetic-resonance imaging (MRI) after partial liver resection characterizing Small-for-Size Syndrome (SFSS), which frequently leads to fatal post-hepatectomy liver failure (PLF). Twenty-two C57BL/6 mice underwent either conventional 70% partial hepatectomy (cPH), extended 86% partial hepatectomy (ePH) or SHAM operation. Subsequent MRI scans on days 0, 1, 2, 3, 5 and 7 in a 4.7T MR Scanner quantified longitudinal and transverse relaxation times, apparent diffusion coefficient (ADC) and the magnetization-transfer ratio (MTR) of the regenerating liver parenchyma. Histological examination was performed by hematoxylin-eosin staining. After hepatectomy, an increase of T1 time was detected being larger for ePH on day 1: 18% for cPH vs. 40% for ePH and on day 2: 24% for cPH vs. 49% for ePH. An increase in T2 time, again greater in ePH was most pronounced on day 5: 21% for cPH vs. 41% for ePH. ADC and MTR showed a decrease on day 1: 21% for ePH vs. 13% for cPH for ADC, 15% for ePH vs. 11% for cPH for MTR. Subsequently, all MR parameters converged towards initial values in surviving animals. Dying PLF animals exhibited the strongest increase of T1 relaxation time and most prominent decreases of ADC and MTR. The retrieved MRI biomarkers indicate SFSS and potentially developing PLF at an early stage, likely reflecting cellular hypertrophy with extended water content and concomitantly diluted cellular components as features of liver regeneration, appearing more intense in ePH as compared to cPH.

Impact of associating liver partition and portal vein ligation for staged hepatectomy (ALPPS) on growth of colorectal liver metastases

BACKGROUND

Associating liver partition and portal vein ligation for staged hepatectomy induces an unprecedented liver hypertrophy and enables resection of otherwise unresectable liver tumors. The effect of associating liver partition and portal vein ligation for staged hepatectomy on tumor proliferation, however, remains a concern. This study investigated the impact of associating liver partition and portal vein ligation for staged hepatectomy on growth of colorectal metastases in mice and in humans.

METHODS

The effect of associating liver partition and portal vein ligation for staged hepatectomy and 90% portal vein ligation on colorectal liver and lung metastases was investigated in mice. In vivo tumor progression was assessed by magnetic resonance imaging, histology, and survival experiments. The effects of associating liver partition and portal vein ligation for staged hepatectomy, portal vein ligation, and control sera on cultures of several colorectal cancer cell lines (MC38 and CT26) were tested in vitro. Additionally, the international associating liver partition and portal vein ligation for staged hepatectomy registry enabled us to identify patients with remaining tumor in the future liver remnant after associating liver partition and portal vein ligation for staged hepatectomy stage 1.

RESULTS

Two and 3 weeks after associating liver partition and portal vein ligation for staged hepatectomy stage 1, portal vein ligation, or sham surgery, liver magnetic resonance images showed similar numbers (P=.14/0.82), sizes (P=.45/0.98), and growth kinetics (P=.58/0.68) of intrahepatic tumor. Tumor growth was not different between the associating liver partition and portal vein ligation for staged hepatectomy and portal vein ligation groups after completion of stage 2. Median survival after tumor cell injection was similar after sham surgery (36 days; 95% confidence interval; 27-57 days), completion of associating liver partition and portal vein ligation for staged hepatectomy (42 days; 95% confidence interval; 35-49 days), and portal vein ligation (39 days; 95% confidence interval; 34-43 days, P=.237). Progression of pulmonary metastases and in vitro cell proliferation were comparable among groups. Observations in humans failed to identify any accelerated tumor growth in the future liver remnant within the regenerative phase after associating liver partition and portal vein ligation for staged hepatectomy stage 1.

CONCLUSION

The accelerated regeneration process associated with associating liver partition and portal vein ligation for staged hepatectomy does not appear to enhance growth of colorectal metastases.

MRI as Primary End Point for Pharmacologic Experiments of Liver Regeneration in a Murine Model of Partial Hepatectomy

RATIONALE AND OBJECTIVES

The study aimed to validate magnetic resonance imaging (MRI)-based liver volumetry as a quantitative measure of hepatic regeneration in mice subjected to partial hepatectomy, in view of routine in vivo pharmacologic studies characterizing compounds aiming to accelerate liver regeneration.

MATERIALS AND METHODS

Partial hepatectomy was performed in male B6 mice (n = 47). Images were acquired in 14.5 minutes from anesthetized and spontaneously respiring animals, without any gating and without administration of contrast material. Some of the mice (n = 6) were treated with 1, 4-bis [2-(3, 5-dichloropyridyloxy)] benzene (TCPOBOP), a synthetic agonist of mouse constitutive androstane receptor, or with the corresponding vehicle (n = 6). Postmortem analyses included total liver weight and histologic Ki67 expression.

RESULTS

A highly significant correlation (R = 0.98, P = 1.5 × 10^-14) was obtained between the MRI-derived liver volumes and the postmortem liver weights in hepatectomized, untreated mice. MRI reliably monitored enhanced murine liver regrowth following treatment with TCPOBOP, as confirmed by comparative hepatocyte proliferation (Ki67 expression) and liver weight analysis (R = 0.96, P = 2 × 10^-6).

CONCLUSIONS

MRI-based monitoring of liver regrowth in mice without the requirement of euthanizing animals at several time points has been established. In comparison to terminal procedures, the number of hepatectomized mice needed to derive a liver (re)growth curve was reduced by a factor of 6. The feasibility of using this imaging approach in pharmacologic studies in the context of liver regeneration has been demonstrated.

Visualization of Vascular and Parenchymal Regeneration after 70% Partial Hepatectomy in Normal Mice

A modified silicone injection procedure was used for visualization of the hepatic vascular tree. This procedure consisted of in-vivo injection of the silicone compound, via a 26 G catheter, into the portal or hepatic vein. After silicone injection, organs were explanted and prepared for ex-vivo micro-CT (µCT) scanning. The silicone injection procedure is technically challenging. Achieving a successful outcome requires extensive microsurgical experience from the surgeon. One of the challenges of this procedure involves determining the adequate perfusion rate for the silicone compound. The perfusion rate for the silicone compound needs to be defined based on the hemodynamic of the vascular system of interest. Inappropriate perfusion rate can lead to an incomplete perfusion, artificial dilation and rupturing of vascular trees. The 3D reconstruction of the vascular system was based on CT scans and was achieved using preclinical software such as HepaVision. The quality of the reconstructed vascular tree was directly related to the quality of silicone perfusion. Subsequently computed vascular parameters indicative of vascular growth, such as total vascular volume, were calculated based on the vascular reconstructions. Contrasting the vascular tree with silicone allowed for subsequent histological work-up of the specimen after µCT scanning. The specimen can be subjected to serial sectioning, histological analysis and whole slide scanning, and thereafter to 3D reconstruction of the vascular trees based on histological images. This is the prerequisite for the detection of molecular events and their distribution with respect to the vascular tree. This modified silicone injection procedure can also be used to visualize and reconstruct the vascular systems of other organs. This technique has the potential to be extensively applied to studies concerning vascular anatomy and growth in various animal and disease models.

Quantification of Hepatic Vascular and Parenchymal Regeneration in Mice

BACKGROUND

Liver regeneration consists of cellular proliferation leading to parenchymal and vascular growth. This study complements previous studies on cellular proliferation and weight recovery by (1) quantitatively describing parenchymal and vascular regeneration, and (2) determining their relationship. Both together are needed to (3) characterize the underlying growth pattern.

METHODS

Specimens were created by injecting a polymerizing contrast agent in either portal or hepatic vein in normal or regenerating livers after 70% partial hepatectomy. 3D image data were obtained through micro-CT scanning. Parenchymal growth was assessed by determining weight and volume of the regenerating liver. Vascular growth was described by manually determined circumscribed parameters (maximal vessel length and radius of right inferior portal/hepatic vein), automatically determined cumulative parameters (total edge length and total vascular volume), and parameters describing vascular density (total edge length/volume, vascular volume fraction). The growth pattern was explored by comparing the relative increase of these parameters to the increase expected in case of isotropic expansion.

RESULTS

Liver volume recovery paralleled weight recovery and reached 90% of the original liver volume within 7 days. Comparing radius-related vascular parameters immediately after surgical resection and after virtual resection in-silico revealed a slight increase, possibly reflecting the effect of resection-induced portal hyperperfusion. Comparing length-related parameters between post-operative day 7 and after virtual resection showed similar vascular growth in both vascular systems investigated. In contrast, radius-related parameters increased slightly more in the portal vein. Despite the seemingly homogeneous 3D growth, the observed vascular parameters were not compatible with the hypothesis of isotropic expansion of liver parenchyma and vascular structures.

CONCLUSION

We present an approach for the quantitative analysis of the vascular systems of regenerating mouse livers. We applied this technique for assessing the hepatic growth pattern. Prospectively, this approach can be used to investigate hepatic vascular regeneration under different conditions.

Selective portal vein injection for the design of syngeneic models of liver malignancy

Liver metastases are the most frequent cause of death due to colorectal cancer (CRC). Syngeneic orthotopic animal models, based on the grafting of cancer cells or tissue in host liver, are efficient systems for studying liver tumors and their (patho)physiological environment. Here we describe selective portal vein injection as a novel tool to generate syngeneic orthotopic models of liver tumors that avoid most of the weaknesses of existing syngeneic models. By combining portal vein injection of cancer cells with the selective clamping of distal liver lobes, tumor growth is limited to specific lobes. When applied on MC-38 CRC cells and their mouse host C57BL6, selective portal vein injection leads with 100% penetrance to MRI-detectable tumors within 1 wk, followed by a steady growth until the time of death (survival ∼7 wk) in the absence of extrahepatic disease. Similar results were obtained using CT-26 cells and their syngeneic Balb/c hosts. As a proof of principle, lobe-restricted liver tumors were also generated using Hepa1-6 (C57BL6-syngeneic) and TIB-75 (Balb/c-syngeneic) hepatocellular cancer cells, demonstrating the general applicability of selective portal vein injection for the induction of malignant liver tumors. Selective portal vein injection is technically straightforward, enables liver invasion via anatomical routes, preserves liver function, and provides unaffected liver tissue. The tumor models are reproducible and highly penetrant, with survival mainly dependent on the growth of lobe-restricted liver malignancy. These models enable biological studies and preclinical testing within short periods of time.

Single- versus dual-console robot-assisted radical prostatectomy: impact on intraoperative and postoperative outcomes in a teaching institution

OBJECTIVE

To compare the outcomes of robotic-assisted laparoscopic prostatectomy (RALP) using a dual versus single-console system in a resident training program using intraoperative, perioperative and postoperative measures.

METHODS

Patients with PCa who underwent RALP prior to and after implementing a dual-console system at an academic institution were reviewed from 2006-2012. All surgeries were performed by a single-faculty surgeon well after the learning curve was established. In all cases, chief residents participated in the surgery and performed progressively more portions. Demographic, intraoperative and pathologic parameters were obtained. Continence and erectile function were assessed at 6 and 12 months. Postoperative complications were graded using the Clavien-Dindo classification. Predictors of outcomes on univariate analysis were included in multivariate logistic or linear models.

RESULTS

Of 381 patients, 185 and 196 underwent single- or dual-console RALP, respectively. There was a significant decrease in mean operative time using the dual-console system (222 vs. 171 min, p < 0.0001) as well as in the incidence of intraoperative complications (8.65 vs. 1.53%, p < 0.0001) and postoperative complications (14.1 vs. 6.63%, p = 0.03.) Complications of Clavien grade ≥3a occurred more frequently with a single-console system (7 vs. 1%, p = 0.003.) Differences persisted when controlling for potential confounders by multivariate regression. Postoperative measures of continence, erectile function and the rate of biochemical recurrence were similar between cohorts.

CONCLUSIONS

When training resident surgeons to perform RALP, a dual-console system may improve intraoperative and perioperative outcomes. The dual-console may represent a safer, more efficient modality for robotic surgical education as compared to a single-console system.